Looks like PBL is offering a special mylar for SM magnets.

It's important to protect the magnet and playfield, but just slapping mylar on top removes one of the coolest effect from the game, the ball fling! Right now I just stuck a tiny bit of mylar at the lower left corner of the magnet, it prevents the ball from hopping but still allowd the magnet to fling the ball into the scoop.

Hopefully this piece still allows for the fling effect.

I'm going to order this next time I put an order into PBL. If anyone is doing an order sooner, let me know pleaze

I heard the mylar gets chewed up on the magnets and not to use it.  I cut my own donuts around the magnet on my IM.